Police Doppler radars need to acquire the patrol speed of the police car in various moving modes to be able to calculate the target speed of a tracked target in either moving, same lane mode or moving, opposite lane mode. In moving, opposite lane mode, the speed of the patrol car must be subtracted from the Doppler closing speed of the target being tracked to correctly calculate the speed of the target. In moving, same lane mode, the speed of the patrol car must be added to the Doppler closing speed found from the radar returns to derive the actual speed of the target. Obviously if the patrol speed is wrong, the calculated speed of the target will be wrong.
In the prior art Doppler radars such as the Applied Concepts Stalker Dual and the Kustom Eagle series as well as other Doppler police radars, the patrol speed was acquired by searching the Fourier components of the Doppler return signal for radar reflections from stationary objects such as billboards, trees, the ground, etc. Usually, the strongest signal component represented a return from the ground or another stationary object. However, that is not always true, and false patrol speed signals could be locked onto and tracked if another signal from, for example, a truck with a large radar cross section is the strongest signal in the spectrum. In other words, the fact that a radar return signal is the strongest signal in the spectrum does not guarantee that it is from a stationary object and represents the patrol speed.
To resolve that ambiguity, radars like the Kustom Eagle series use complicated digital signal processing to examine the shape of the Fourier component spectrum around the peak which the radar thinks is the patrol speed return from a stationary object. Patrol speed returns typically have an asymmetrical shape around the peak, and the Eagle series radars took advantage of this fact by examining the shape of the spectrum around each peak which is suspected of being a patrol speed return to determine if the characteristic asymmetric shape was present. This complicates the software unnecessarily and increases the cost to design it, and is not foolproof anyway. Further, it is hardly worth using this methodology since it does not help in many cases.
Another problem with deriving the patrol speed from radar returns is that when the patrol car comes to a stop. Typically, after a patrol speed return from a stationary object is found, the radar tracks that return even at a lower amplitude to maintain continuity due to traffic in the antenna beam that reduces the reflected return from stationary objects. Problems arise however when the police vehicle comes to a stop, and the lock on the patrol speed return is lost. The software of these prior art radars goes into a search mode when lock is lost on the patrol speed return in an attempt to find a new patrol speed return. The radar will then often lock onto a return from another vehicle instead of a return from a stationary object and will conclude that the speed of that other vehicle is the patrol speed.
This is called xe2x80x9cshadowingxe2x80x9d and frequently occurs when a patrol car has its radar operating in moving mode and pulls to a stop at a stop sign behind another car. When the other car takes off from the stop sign, the radar will often lock onto the speed of that car as the patrol speed and fail to lock onto the actual patrol car speed when the patrol car starts moving again.
When the patrol car is stopped, the radar needs to be manually switched into stationary mode to prevent this xe2x80x9cshadowing problemxe2x80x9d from happening. When the patrol car starts moving again, the radar must be manually switched back to moving mode. This is inconvenient to the officer, and if he forgets, a bad patrol speed can be locked which leads to errors in the tickets he writes.
Early attempts to use speedometer interfaces to find patrol speed are represented by U.S. Pat. No. 4,335,382 to Brown and assigned to Decatur. That patent taught a traffic radar system in which the speed of a target vehicle is determined by measuring the difference in frequency between a component of a doppler signal which has a frequency proportional to the relative speed of the target vehicle and a moving patrol vehicle and a reference signal having a frequency proportional to the speed of the patrol vehicle. The reference signal is developed from a tachometer device which generates a periodic signal having a frequency proportional to the rotational speed of a vehicle wheel and phase-locked loop arrangements including adjustable dividers are provided for locking an oscillator to the tachometer signal and generating a reference signal at a frequency proportional to the actual speed of the patrol vehicle. For calibration, a component of the doppler signal produced from reflections from stationary objects is used.
More recently, Kustom Signals obtained U.S. Pat. No. 6,023,236 for Speedometer Assisted Patrol Speed Search For DSP Traffic Radar. This patent demonstrates an operator sequence to synchronize the speedometer input to the radar return. Synchronization is typically done upon initial installation or when the radar is moved to another vehicle.
In U.S. Pat. No. 5,525,996 owned by the assignee of the present patent application and covering the Stalker Dual, a method of automatically rejecting the patrol speed determined from the ground return on the basis of too high a difference from a speed determined from a speedometer interface was taught. Specifically, the ""996 patent taught,
xe2x80x9cThe DSP also has an optional speedometer interface 424 in some embodiments through which the DSP can read the actual patrol car speed for purposes of comparing this speed to the xe2x80x9cpatrol speedxe2x80x9d derived from the radar returns of stationary objects. The software of Appendix 1 does not use the actual speedometer speed as the patrol speed because this is not accurate enough. Instead, the patrol speed is derived from the Doppler shifted radar returns from the ground. This is done by using the strongest radar return over time and assuming this is the return from the ground. Basically, the ground return is usually the strongest radar return although when targets get close, their returns become stronger for a short time than the ground return but then the target return goes away altogether. The software therefore assumes that the strongest return over an interval which is longest enough to distinguish over transitory target returns, is the return from which the patrol speed is derived. The software also includes the ability to compare the calculated patrol speed from the return selected in the above described manner to the speed read from the speedometer, and, if the difference is greater than 3 MPH, to discard the calculated patrol speed and re-calculate it from a different radar return.
None of these prior art attempts was completely satisfactory. Therefore, a need has arisen for a process and apparatus to use speedometer pulses to steer the DSP search for a patrol speed which automatically calibrates itself on every powerup and does not need to have a speedometer input to operate so that it can be moved from a car with a speedometer output to a car without a speedometer output with no setup needed.
The invention is a process and apparatus for using speed pulses from a speedometer to steer a search by a digital signal processor on an Fourier transform based police Doppler radar to find the correct patrol speed from stationary object returns. The system automatically calibrates itself (finds the correct ratio between speed sensor output frequency and true vehicle ground speed) so that it can be moved easily from one car to another with different speed versus frequency characteristics of their speedometers. No human input is needed of any sort to complete this automatic calibration process.
Further, in some embodiments, the radar can also operate like the prior art radars to find the ground speed without any speedometer input at all. If installed in a car with no speedometer output coupled to the radar, this fact is automatically sensed, and the police Doppler traffic radar operates as all prior art DSP Doppler radars and finds the ground speed without any software window that limits the ground speed search.
No setup is needed when moving the unit from a car with no speedometer output to a car with a speedometer output. Calibration of speedometer frequency to vehicle speed is automatic and neither requires operator entered configuration data nor any other operator input.
The system also senses when the patrol car is moving and automatically switches the radar from stationary mode to moving mode. When the car stops, the system automatically switches the radar back from moving mode to stationary mode to prevent the shadowing problem.
A broad description of the calibration process of the invention is:
a) reading at least one signal frequency output from a vehicle speed sensor to a police Doppler traffic radar, and obtaining from the Doppler police traffic radar what the traffic radar thinks is the correct ground speed calculated from the reflected signal from a stationary object at the time the signal frequency from said vehicle speed sensor is read;
b) repeating the process of step a) multiple times and storing the data;
c) automatically determining the reliability of the ratio without any input from a human being needed by evaluating over time one or more factors which are indicative that said police Doppler traffic radar has locked onto and is tracking the correct ground speed including at least persistence of the same ratio for a plurality of different ground speeds obtained from said police Doppler traffic radar; and
d) when the reliability of the ratio has been adequately established, storing the ratio so determined to be accurate in a memory for use by said police Doppler traffic radar in calculating the speed of moving targets when searching in moving modes.
The process carried out by the invention to use the ratio determined by the automatic calibration process to steer the ground speed search by the radar is comprised of the following steps:
a) when the reliability of the ratio has been adequately established, storing the ratio so determined to be accurate in a memory for use by said police Doppler traffic radar in calculating the speed of moving targets when searching in moving modes;
b) calculating an approximate ground speed by reading said ratio stored in said memory and multiplying said ratio times the frequency received from a vehicle speed sensor;
c) using said approximate ground speed calculated in step b) to establish and limit the range of speeds to be searched by said police Doppler traffic radar for a ground speed; and
d) searching the range of speeds established in step c) for the strongest reflected radar signal and setting the ground speed equal to the speed which corresponds to the frequency of said strongest reflected radar signal.
The overall process carried out by the invention is as follows:
a) automatically determining whether a police traffic radar is installed in a Vss equipped car, and monitoring for any nonzero frequency at an input of said police Doppler traffic radar that would be coupled to a speed sensor if the vehicle was Vss equipped;
b) if a conclusion is drawn that the traffic radar is being used in a vehicle which is not Vss equipped, finding ground speed in a conventional manner without use of any ground speed search window steered using data derived from a vehicle speed sensor;
c) if a conclusion is drawn that the traffic radar is installed in a vehicle which is Vss equipped, monitoring the frequency received from the vehicle speed sensor, and causing automatic switching of said police traffic radar from moving mode to stationary mode when the vehicle is not moving and automatic switching from stationary mode to moving mode when the vehicle begins to move; and
at least once after power is first supplied to said police Doppler traffic radar and said radar is turned on, performing the following steps to implement an automatic calibration process to determine the correct ratio between frequency supplied from said speed sensor:
d) reading at least one signal frequency output from a vehicle speed sensor to a police Doppler traffic radar, and obtaining from the police Doppler traffic radar what the traffic radar computes is the correct ground speed calculated from the reflected signal from a stationary object at the time the signal frequency from said vehicle speed sensor is read;
e) repeating the process of step d) multiple times and storing the data;
f) automatically determining the reliability of the ratio without any input from a human being needed by evaluating over time one or more factors which are indicative that said police Doppler traffic radar has locked onto and is tracking the correct ground speed including at least persistence of the same ratio for a plurality of different ground speeds obtained from said police Doppler traffic radar;
g) when the reliability of the ratio has been adequately established, storing the ratio so determined to be accurate in a memory for use by said police Doppler traffic radar in calculating the speed of moving targets when searching in moving modes; and
after said ratio has been determined to be reliable, performing the following steps each time a moving mode search is to be performed by said police Doppler traffic radar:
h) calculating an approximate ground speed by reading said ratio stored in said memory and multiplying said ratio times the frequency received from a vehicle speed sensor;
g) using said approximate ground speed calculated in step h) to establish and limit the range of speeds to be searched by said police Doppler traffic radar for a ground speed return; and
g) searching the range of speeds established in step g) for the strongest reflected radar signal and setting the speed that corresponds to the frequency of said strongested reflected radar signal as the ground speed for use in calculating the actual speed of a moving target whose relative speed was determined in a moving mode target search.
The apparatus of the invention comprises any digital signal processing based Doppler traffic radar programmed to perform the processes described above.